This study will continue to extend work on the distribution, proliferation and infectivity of fleas and lice infected with various species of pathogenic rickettsiae. The insects will be fed blood meals containing R. mooseri or R. prowazeki and the microorganisms will be examined by electron microscopy at various stages of infection throughout the life of the insect vectors. Rickettsiae will also be studied in the host mice, tissue culture cells, and in the chick yolk sac innoculum. In addition to transmission electron microscopical study of thin sections, freezefractures of infected fleas and lice will be made to examine and compare these observations. Some earlier observations on R. coneri and R. tsutsugamushi will also be extended. Localization of labeled antibodies to various species of pathogenic rickettsia by electron microscopy will be initiated.